1. Field of the Invention
The present invention relates to a fuel cell system for producing electricity by using an electrochemical reaction.
2. Description of the Background Art
It is known that, when shutting down a fuel cell system with air remaining on a cathode side, a catalyst forming a cathode catalyst layer is deactivated, resulting in electrode performance degradation. More specifically, in an exemplary case where platinum catalyst particles serving as a catalyst are held by carbon particles, the carbon particles react with oxygen present in incoming air, then turn into carbon monoxide and disappear, while the platinum catalyst particles are separated from the carbon particles and are flocculated together, so that the catalyst loses conduction with the cathode electrode and thus loses its catalytic function. To avoid such degradation, a shutdown method is known which includes shorting the output of a fuel cell to a resistor when stopping electricity production, stopping the supply of air to the cathode, and completely consuming air remaining on the cathode (cf. Japanese Patent Application Laid-Open No. 11-26003 (1999)).
Shorting the fuel cell output to the resistor while stopping the supply of oxygen to the cathode mainly produces the following reactions (1) and (2) on the cathode:2H++(½)O2+2e−→H2O  (1)2H++2e−→H2  (2)
The reaction (1) is dominant when there is oxygen of relatively high concentration for some time after the start of cutting off the supply of air, and the reaction (2) is dominant after the concentration of oxygen is lowered. The reaction (2) produces hydrogen on the cathode, causing the potential around the catalyst to drop, which prevents oxidization of the catalyst. Therefore, shutting down and storing the fuel cell in this state is a countermeasure against the above-described degradation.
Experiments conducted by the inventors of the present invention have revealed that shorting the fuel cell output to a resistor to bring both anode and cathode into an atmosphere of hydrogen and then shutting down and storing the fuel cell with the inlet and outlet of each of the anode and cathode closed not only prevents catalyst degradation, but also advantageously allows a degraded catalyst to be activated again (a publicly-unknown technique). This is considered because, when a catalyst is covered with such an oxide that reduces the catalytic powers, the oxide is reduced by hydrogen. Accordingly, consideration will be given to an application of this publicly-unknown shutdown and storage method to a system provided with a reformer for producing a fuel to be supplied to a fuel cell, from a raw fuel.
To apply the above-mentioned publicly-unknown shutdown and storage method of maintaining both anode and cathode in an atmosphere of hydrogen to a system for supplying fuel from a reformer to an anode, fuel needs to be supplied from the reformer to the anode until oxygen on the cathode is consumed to lower the concentration of oxygen, so that hydrogen is produced on the cathode. In this case, when the output is connected to a resistive load upon stopping the supply of air similarly to the background art, stack current decreases in proportion to stack voltage, reducing the amount of consumption of hydrogen on the anode, so that the amount of hydrogen contained in an anode off-gas increases. Further, it takes relatively much time to remove the residual oxygen on the cathode, which increases the operating time of the reformer after stopping the supply of air. Therefore, an excessive amount of hydrogen contained in the off-gas, which is a fuel, is burned with an off-gas burner of the reformer for a long while, causing a problem of abnormal temperature rise in the reformer.